1. Field of the Invention
The present invention relates to apparatus and techniques for detecting the temperature differential existing around various electrical power equipment and enclosures.
2. Description of Related Art
Residential, commercial, and industrial buildings typically have electrical panels in the form of a box or covered recess that may contain circuit breakers, power contactors, relays, fuses or other equipment designed to deliver or route primary utility current to locations in the building. Failure of such electrical equipment is typically preceded by a temperature increase. The temperature increase can be caused by excessive current, unbalanced load, oxidation or corrosion at contact surfaces, lossy contacts that generate heat, arcing, etc.
Measuring a temperature increase within an electrical panel is complicated by the fact that the temperature rise can be caused by equipment defects, or simply by a rise in the ambient temperature. For this reason equipment for detecting or anticipating equipment failure will measure a temperature difference, that is, the temperature at a piece of monitored equipment relative to ambient.
A disadvantage with measuring these temperature differences is the complexity associated with the equipment capable of performing such measurements. For example, monitoring the temperature inside a power panel will often require a skilled electrician who is able to remove the panel cover while energized and safely install a temperature sensor as well as wiring that leads to the outside of the power panel. Also, an external temperature sensor must be mounted outside the panel at a position appropriate for measuring ambient temperature. This external sensor must then be wired to a monitoring circuit that can perform the differential analysis and provide an appropriate warning signal. Being relatively complex, such systems often consume a fair amount of power and are therefore often connected to utility power lines, which adds to the complexity of the installation.
In U.S. Pat. No. 4,901,060 a temperature sensitive thyristor 34 energizes warning element 20 when high temperature is sensed at device 18, which is illustrated as a standard socket for house current. Warning element 20 can be a light or a flasher. A temperature reference can be provided to thyristor 34 by diode 64 of device 62, which is spaced a distance d from the thyristor 34.
In U.S. Pat. No. 6,470,735 the temperature of lubricant in an axle housing is measured by sensor 26 and compared to the ambient temperature measured by sensor 30. An excessive temperature difference indicates a probable need for service and can illuminate light 36.
In U.S. Pat. No. 5,541,803 a temperature difference is sensed by a sensor conductor and a reference conductor. In the embodiment of FIG. 17 a sensor conductor 174 is routed inside an appliance to compare the temperature inside the appliance to the temperature on the outside of the appliance. Power to the appliance is interrupted in response to excessive internal temperature. LEDs 32 and 33 indicate the status of the system as either “tripped” or “on.”
In U.S. Pat. No. 6,707,652 a terminal of a circuit breaker (or the like) may glow hot if it has a poor, high resistance connection. Temperature sensing diodes 8 and 10 can sense the different temperatures and the magnitude of the difference determines whether the comparator will trip a circuit breaker coil.
In U.S. Pat. No. 5,982,849 a device adhesively attached to an x-ray tube can operate a blinker 11 when resistance sensor 7 detects a high temperature.
In U.S. Pat. Nos. 5,847,653 and 6,060,990 a heat alarm is attached to the face of an electrical panel by magnets or otherwise. A bimetallic switch senses temperature reaching 135° F. to illuminate an LED and operate an audible alarm.
In U.S. Pat. No. 5,461,367 a bimetallic temperature sensor is mounted inside an electrical panel to produce an audible alarm when the panel temperature exceeds 135° F.
In FIG. 4 of U.S. Pat. No. 4,331,888 temperature sensitive transistors Tr1 and Tr2 have different thermal time constants. Accordingly, a differential voltage will be produced in response to a rapid temperature increase. This differential voltage can trigger a temperature sensitive thyristor that can also trigger in response to high temperatures, without regard to the differential voltage. Once triggered, the thyristor operates an alarm 7.
In U.S. Pat. No. 4,406,550 temperature sensors 10 and 11 are applied to differential comparator 30 to display a temperature difference on display 38. The device is described as useful for monitoring temperature differences at different positions on a diesel engine, a furnace, a solar collector, or at different positions around a building (including the inside and outside of the building). If the temperature difference exceeds a certain positive threshold the system activates an alarm 55. If the temperature difference exceeds a certain negative threshold, alarm 56 is activated instead.
In U.S. Pat. No. 6,359,565 the temperatures at various locations on several electronic cards are monitored by sensors mounted on those cards. The measured temperatures are compared electronically to the ambient temperature measured at a fan. Components having a high temperature differential over the ambient temperature are deemed to be malfunctioning and the operator is given an alarm signal and is offered information in the form of a thermal map of the system.
In U.S. Pat. No. 5,081,359 a differential thermal sensor using thermopiles can detect temperature differences along a patient's spine, or in various industrial processes.
In U.S. Pat. No. 4,608,565 an indoor/outdoor thermometer uses a radio frequency connection to avoid cutting a hole through a building. Temperature is displayed by a numeric display.
In U.S. Pat. No. 3,688,295 a thermocouple for measuring brake temperature is compensated so that changes in ambient temperature due to cold or hot weather do not affect the temperature measurement at the hot junction of the thermocouple. As temperature increases the system illuminates a warning light and then an overheat light.
In U.S. Pat. No. 4,188,623 the hot junction of the thermocouple is placed near an automobile's catalytic converter. If the thermocouple measures a high temperature, reed switch 20 closes to illuminate light 26. The cold junction is responsive to the “circumferential temperature” but a temperature sensitive diode or thermistor is used to compensate for or cancel out the effects of the circumferential temperature.
In U.S. Pat. No. 5,229,612 a thermopile is coupled with a thermocouple so that a measured temperature is referenced to a remote temperature sensed by the thermocouple. In some embodiments separate thermopiles measure temperatures at spaced positions to develop a differential temperature measurement.
In U.S. Pat. No. 6,429,777 thermistors are mounted in junction boxes throughout a building. The measured temperatures can be displayed on a panel at a central station.
In U.S. Pat. No. 4,922,230 a reference temperature signal is initially set at startup and is allowed to vary slowly if temperature increases. The difference between this reference temperature and the actual temperature as sensed by the sensor 10 is used to trigger an alarm, basically when the sensed temperature is rising so fast as to indicate the outbreak of the fire.
In U.S. Pat. No. 3,753,194 a pair of thermostats produces a signal when the temperature falls between an upper and lower limit.
Accordingly, there is a need for an improved device for measuring temperature differences associated with electrical equipment that avoids the shortcomings and complexities of the prior art.